Make the brownie batter and combine most of the graham cracker pieces (about 6 pieces) into the batter.
Spread it evenly on a 9x9-inch pan.
Bake the brownies according to the package instructions.
Add the marshmallows and remaining graham crackers on top.
Broil it on low until marshmallows are lightly toasted, about 2 minutes. Watch it closely as you do this.
While still hot, top with chocolate pieces.
Let it set for at least an hour in the fridge before cutting.
Notes
Tips for best brownies:
Bring eggs to room temperature and lightly beat them to incorporate them well without overmixing.
Grease and line the pan with parchment paper to prevent the brownies from sticking.
To avoid overbaking, check the brownies with a toothpick 2 minutes before they are done. Look for several separate crumbs for fudgy brownies and less for cakey brownies.
Cool the brownies before slicing. Refrigerating them improves their texture.